From the standpoint of preventing damage to the environment by sulfur oxides and from the standpoint of the increasing rigour of exhaust gas regulations for cars, demand has grown in society for a reduction in the sulfur content of car fuels. Present regulations relating to sulfur content have become increasingly rigorous in stages, and the sulfur content of the diesel oils currently on the market is limited by a JIS standard to not more than 10 ppm by mass.
In general, to reach a sulfur content of not more than 10 ppm by mass it is possible to reduce the petroleum fractions having distillation characteristics corresponding to the composition, that is to the diesel oil, by, for example, using methods of hydrodesulfurisation such as disclosed in Japanese Laid-open Patents 2000-109860 and 2008-266420, and at the same time the colour is improved by the addition of heavy fractions such as polycyclic aromatics contained in the raw material, and an almost colourless and transparent diesel oil is obtained for the market. The colour itself has no impact on the performance of diesel engines used in cars, but given that deterioration of the colour gives an impression that there has been deterioration of the diesel oil's characteristics, it makes sense for the technology to improve the colour as well, e.g. mainly by the methods of hydrodesulfurisation as mentioned above.
It is known that in diesel oils, peroxides are formed by oxidation and cause deterioration of elements of the vehicle fuel system (rubber and metal elements). They also give rise to problems such as clogging of fuel filters through the formation of sludge, and poor sliding of injection pumps. In consequence, whilst the property of oxidative stability of diesel oil in the present JIS standard has not been standardised, the oxidative stability of diesel oil is an important indicator, and demand calls for diesel oils with excellent oxidative stability. Furthermore, in diesel engines of recent years, common rail-type fuel injection devices have been adopted as a way of reducing particulates (or particulate matter: referred to as “PM” hereinafter), but in such common rail-type fuel injection devices, at the structural level, surplus fuel fed in under pressure to the injectors but not sprayed into the combustion chambers is returned to the fuel tank via a return circuit. This fuel returned to the fuel tank (return fuel) is at an elevated temperature, and so oxidation of the diesel oil inside the fuel tank is promoted, which means that there is even more of a requirement to enhance the oxidative stability of the diesel oil. Also, because of the increasing rigour of exhaust gas standards in recent years, it can be expected that fuel injection pumps will work at ever higher pressures, and the temperature of the return fuel will rise, too, which means that the importance of oxidative stability will further increase.
However, in recent years, it has become clear that, in diesels oil in which the sulfur component has been reduced to not more than 10 ppm by mass, the oxidative stability also ends up being reduced. Countermeasures against the reduction of oxidative stability, for example by using anti-oxidants, have generally been common, but heavy use of additives leads to increased costs, and other problems may also arise because of separation of additives under certain conditions. Methods have therefore been proposed to enhance the oxidative stability of the reduced-sulfur composition itself without using additives. For example, Japanese Laid-open Patents 2006-137919, 2006-137920, 2006-137921 and 2006-137922 purport to have discovered that naphthene benzenes and fluorenes have a detrimental effect on oxidative stability, and that naphthalenes effect an improvement in oxidative stability, and also therefore propose techniques for making a low-sulfur diesel oil composition which has excellent oxidative stability without adding any anti-oxidant, by regulating the sum of the naphthene benzenes and fluorenes to not more than 8.0% by volume and the naphthalenes to 0.5 to 3.0% by volume, thereby regulating the range of the oxidation stability index.
Also, in Japanese Laid-open Patents 2008-144156 and 2008-144158, techniques have been disclosed for making a low-sulfur diesel oil composition which has excellent oxidative stability without adding any anti-oxidant, by regulating the total contents of styrenes and dienes of specific carbon numbers, being substances that have a detrimental effect on oxidative stability, and of condensed bicyclic and tricyclic aromatic hydrocarbons, being substances to improve oxidative stability, and thereby bringing the oxidation index to within a suitable range.
However, in the technology as disclosed in the above-mentioned patent literature, although it has been shown that substances such as naphthene benzenes, fluorenes, styrenes, dienes, condensed bicyclic and tricyclic aromatic hydrocarbons have some effect on the oxidative stability of diesel oil compositions, there are many unclear points as regards the effect they each have on oxidative stability. Also, in the prior art, oxidative stability has been evaluated at 100° C. or 115° C., but since the fuel temperatures in the newest fuel injection pumps are even higher, the present situation is that it is uncertain to what extent these substances contribute to oxidative stability at high temperatures.